Estimate the amount of plywood or OSB needed for sheathing a roof. The calculation is based on that the Plywood or OSB (Oriented Strand Board) sheets are of a 4' x 8' size. One sheet = 32 ft²

This basically calculates the number of sheets needed for a Gable roof.
The square footage of a Hip roof area is identical to the square footage of a Gable roof of the same measurements. If you are calculating the number of sheets needed for a Hip roof then for the value of the gable overhang use the eaves overhang dimension, assuming it is the same all around.
The amount of waste on a Hip roof will be higher than for sheathing a Gable roof and even more so if you are using OSB sheathing. OSB sheathing has a rough side and a smooth side. You always want to make sure the rough side is up with OSB sheathing. Never install the OSB sheathing with the smooth slippery side up!

Instructions

1. building length in feet and inches
2. building width, the "gable side" wall in feet and inches
3. gable overhang in inches
4. eaves over hang in inches
5. waste factor in a percentage preset @ 10%

The Results

1. roof area in square feet
2. number of sheets needed based on exact square footage of the roof
3. the number of sheets the waste percentage will add
4. Number of sheets needed including the waste